Crystal growth method for nitride semiconductor having a multiquantum well structure

ABSTRACT

A crystal growth method for nitride semiconductors, including the steps of growing a first semiconductor layer containing In x Ga 1-x N (0&lt;x≦1) on a substrate, with the use of a first carrier gas formed with an inert gas; growing a second semiconductor layer containing In y Ga 1-y N (0≦y&lt;1, y&lt;x) on the first semiconductor layer, with the use of a second carrier gas containing the inert gas and H 2  gas, an amount of the H 2  gas being smaller than an amount of the inert gas; and growing a third semiconductor layer containing In z Ga 1-z N (0≦z&lt;1, z&lt;x) on the second semiconductor layer, with the use of a third carrier gas containing the inert gas and H 2  gas, an amount of the H 2  gas in the third carrier gas being a smaller than the amount of H 2  gas in the second carrier gas.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a Continuation of U.S. application Ser. No.12/874,594, filed Sep. 2, 2010, which claims priority to Japanese patentapplication JP 2010-032630, filed on Feb. 17, 2010.

FIELD

Embodiments described herein relate to a crystal growth method fornitride semiconductors.

BACKGROUND

Light emitting diodes (LEDs) using nitride semiconductor materials havelow toxicity, and characteristically have high efficiencies and longlives. Such LEDs are used in display devices, signals, and lightings,and are available as products in the market. Laser diodes (LDs) usingnitride semiconductor materials are used in light sources for performingwriting on and reading from high-density memory disks, and are availableas products in the market.

By applying a multiquantum well (hereinafter also referred to as MQW)structure of In_(x)Ga_(1-x)N to an active layer of a nitridesemiconductor, a high-efficiency light emitting device having a highluminance can be formed. In a typical quantum well structure, a 1.5 nmto 5 nm material mainly containing In_(x)Ga_(1-x)N is used for thequantum well layer, and GaN is mainly used for the barrier layer, toachieve high-luminance light emission. Particularly, the luminousefficiency and emission wavelength can be varied by changing thecomposition ratio x of In as a group III element forming the quantumwell layer and changing the thickness of the quantum well layer. Forexample, by increasing the composition ratio x of In in the quantum welllayer, light emission of longer wavelengths can be achieved.

To form the above described MQW, it is necessary to successively formIn_(x)Ga_(1-x)N layers containing different amounts of In from oneanother. To grow GaN by conventional metal organic vapor phase epitaxy(MOVPE) under optimum conditions, the growth temperature should be inthe range of 1000° C. to 1200° C., and a H₂ gas should be used as thecarrier gas. Meanwhile, to grow InN under optimum conditions, thetemperature should be in the range of 500° C. to 650° C., and a N₂ gasshould used as the carrier gas. There are great differences in growthconditions between the two, and there are many problems in the crystalgrowth of In_(x)Ga_(1-x)N mixed crystals. For example, since the optimumgrowth temperature of InN is much lower than the optimum growthtemperature of GaN, an even lower temperature is required in the crystalgrowth of an In_(x)Ga_(1-x)N layer having a higher In composition ratio.In a case where a GaN layer to be a quantum well layer requiring a hightemperature is successively grown after an In_(x)Ga_(1-x)N layer to be aquantum well layer such a low temperature is grown, many pits (holes)and clusters (also called protrusions or inclusions) are formed in thesurface, and many defects are induced. As a result, a high-efficiencylight emitting device having a high luminance cannot be obtained. Thisproblem becomes even more serious in cases where an even lowertemperature is required for the growth or where the composition ratio ofIn needs to be made higher. For example, in a case where a MQW thatemits green, yellow, orange, or red light is grown, which has a longerwavelength than blue light, this problem becomes prominent.

Meanwhile, there has been a known technique by which an InGaN quantumwell layer is grown at a growth temperature of 700° C. with the use of aN₂ gas as the carrier gas, the growth temperature is then raised, and H₂gas is added to the carrier gas of N₂ at a growth temperature of 900°C., thereby, a GaN barrier layer is grown.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional diagram of a semiconductor light emittingdevice that is formed according to a crystal growth method of anembodiment;

FIG. 2 is a cross-sectional diagram showing a quantum well layer that isformed according to the crystal growth method of an embodiment;

FIG. 3 is a diagram showing the sequences of the materials, carriergases, and temperatures in the crystal growth of a MQW layer accordingto an embodiment;

FIG. 4 is a diagram showing the sequences of the materials, carriergases, and temperatures in the crystal growth of a MQW layer accordingto Comparative Example 4;

FIG. 5 is a photograph taken when the surface of the MQW layer formedaccording to the method of an embodiment was observed with an atomicforce microscope;

FIG. 6 is a photograph taken when the surface of the MQW layer formedaccording to the method of Comparative Example 1 was observed with anatomic force microscope;

FIG. 7 is a photograph taken when the surface of the MQW layer formedaccording to the method of Comparative Example 2 was observed with anatomic force microscope;

FIG. 8 is a photograph taken when the surface of the MQW layer formedaccording to the method of Comparative Example 3 was observed with anatomic force microscope;

FIG. 9 is a photograph taken when the surface of the MQW layer formedaccording to the method of Comparative Example 4 was observed with anatomic force microscope; and

FIG. 10 is a diagram showing the results of PL measurement carried outon the embodiment and Comparative Examples 1 through 4.

DETAILED DESCRIPTION

Certain embodiments provide a crystal growth method for nitridesemiconductors, including: growing a first semiconductor layercontaining In_(x)Ga_(1-x)N (0<x≦1) on a substrate at a first growthtemperature, with the use of a first carrier gas formed with an inertgas; growing a second semiconductor layer containing In_(y)Ga_(1-y)N(0≦y<1, y<x) on the first semiconductor layer at a second growthtemperature higher than the first growth temperature, with the use of asecond carrier gas containing the inert gas and H₂ gas, an amount of theH₂ gas being smaller than an amount of the inert gas; and growing athird semiconductor layer containing In_(z)Ga_(1-z)N (0≦z<1, z<x) on thesecond semiconductor layer at the second growth temperature, with theuse of a third carrier gas containing the inert gas and H₂ gas, anamount of the H₂ gas in the third carrier gas being a smaller than theamount of H₂ gas in the second carrier gas.

Before an embodiment is described, the embodiment is outlined in thefollowing.

The inventors made intensive studies on a crystal growth method formultiquantum wells (MQW), to prevent enlargement of pits and restraingeneration of clusters. As a result, the inventors made the followingdiscoveries about a method for growing a nitride semiconductor layer ofelements serving as a barrier layer that mainly contained Ga andcontained no In or a small amount of In (hereinafter referred to as theGaN layer), and a nitride semiconductor layer of elements serving as awell layer that contained more In than that of the GaN layer, and mainlycontained In and Ga (hereinafter referred to as the InGaN layer).

1. Clusters are removed by adding H₂ gas only in the initial stage offormation of the GaN layer.

2. After that, the GaN layer is grown at a higher temperature than thegrowth temperature of the InGaN layer without the addition of H₂ gas tothe carrier gas. The temperature is then lowered again, and the InGaNlayer is grown, to obtain a flat crystal surface.

The inventors found that a MQW with excellent crystal quality could beformed by repeating those procedures to crystal-grow the MQW, and alight emitting device having a high luminance could be manufactured.

Hereafter, a crystal growth method for nitride semiconductors accordingto an embodiment will be described with reference to FIG. 1. A nitridesemiconductor that is grown by the crystal growth method of theembodiment is a semiconductor light emitting device having a MQWstructure, and FIG. 1 is a cross-sectional diagram of the semiconductorlight emitting device. This semiconductor light emitting device has astructure in which a GaN buffer layer 2, a GaN layer 3, a stack film 4having In_(0.08)Ga_(0.92)N layers and GaN layers alternately stacked, aSi-doped n-type GaN layer 5, a MQW layer 6 having In_(0.18)Ga_(0.82)Nlayers and GaN layers alternately stacked, a Mg-doped p-type AlGaN layer7, a Mg-doped p-type GaN layer 8, and a p-type GaN layer 9 doped with Mgat a high density are stacked in this order on a sapphire substrate 1having a (0001) upper plane. Each of the semiconductor layersconstituting this semiconductor light emitting device is grown by MOVPE(Metal Organic Vapor Phase Epitaxy). As the source and dopant materialsof the atoms forming each of the semiconductor layers, the following rawmaterials can be used.

Examples of Ga raw materials include TMGa (trimethylgallium) and TEGa(triethylgallium). Examples of In raw materials include TMIn(trimethylindium). Examples of Al raw materials include TMAl(trimethylaluminum). Examples of N raw materials include NH₃ (ammonia).Examples of Si raw materials include SiH₄ (monosilane). Examples of Mgraw materials include Cp₂Mg (cyclopentadienylmagnesium).

First, thermal cleaning is performed on the sapphire (0001) substrate 1at a susceptor temperature of 1100° C. The susceptor temperature is thenlowered to 500° C., and the GaN buffer layer 2 is grown on the sapphiresubstrate 1. After the susceptor temperature is raised to 1120° C., theGaN layer 3 is grown at a high temperature. Other than a sapphire (0001)substrate, a SiC substrate or a GaN substrate may be used as thesubstrate 1. It is also possible to use another plane of a sapphiresubstrate, other than the (0001) plane.

After that, the carrier gas is switched from a H₂ gas to a N₂ gas, andthe susceptor temperature is lowered to 850° C. As a buffer layer ofactive layers, the stack film 4 having a stack structure formed byrepeating ten cycles of stacking of an In_(0.08)Ga_(0.92)N layer and aGaN layer is grown. The Si-doped n-type GaN layer 5 is then grown at thesame temperature. It is preferable to have a stack structure that isformed by stacking an InGaN layer and a GaN layer several times and isused as a buffer layer of active layers. However, the same effects asabove can be achieved without such a stack structure, as long as anactive layer is grown on a nitride semiconductor.

After that, the MQW layer 6 having a stack structure formed by repeatingfour cycles of stacking of an In_(0.18)Ga_(0.82)N layer and a GaN layeris grown. The carrier gas is then switched from a N₂ gas to a H₂ gas,and the Mg-doped AlGaN layer 7 is grown at 1020° C. At the same growthtemperature, the Mg-doped GaN layer 8 and the GaN layer 9 doped with Mgat a high density are sequentially grown.

Referring now to FIGS. 2 and 3, the procedures for growing the MQW layer6 according to the crystal growth method of the embodiment are describedin greater detail. FIG. 2 is a cross-sectional diagram showing the MQWlayer 6, and FIG. 3 is a diagram showing the sequences of each material,each carrier gas, and each temperature in the crystal growth of the MQWlayer 6.

In the crystal growth of the MQW layer 6, the flow rate of N₂ gas is20400 sccm, the flow rate of NH₃ is 18600 sccm, and the total flow rateof gases to be supplied into the reacting furnace is maintained at 39000sccm, for example. The flow rate of N₂ gas is partially supplied to ababbler such as TMGa or TMIn. As the carrier gas, it is preferable touse a N₂ gas that is an inert gas, but an Ar or He gas may be used,instead of a N₂ gas.

First, the susceptor temperature is set at 850° C., and TMGa and NH₃ aresupplied as materials into the reacting furnace, to grow the Si-dopedn-type GaN layer 5 to be the base layer. After that, the crystal growthis suspended, and the susceptor temperature is lowered to 730° C., togrow a GaN layer 11 of 3 nm in film thickness. At this point, thesupplies of the raw material gases of TMGa and NH₃ and the carrier gasof N₂ into the reacting furnace are maintained, without a halt. As shownin step S1 of FIG. 3, at the susceptor temperature of 730° C., anIn_(x)Ga_(1-x)N (0<x≦1) layer 12 of 2.5 nm to serve as a quantum welllayer is grown by further adding a raw material gas of TMIn to thesupplies of the raw material gases of TMGa and NH₃ and the carrier gasof N₂. The composition ratio x in the quantum well layer varies with adesired emission wavelength, but is preferably 0.5 or smaller. This isbecause a growth of an In_(x)Ga_(1-x)N layer having x larger than 0.5has large lattice mismatch between the layer and the base GaN layer, andtends to cause the problem of relaxation of strain. The GaN layer 11 maynot be grown prior to the growth of the In_(x)Ga_(1-x)N layer 12. Inthat case, however, it takes time to lower the growth temperature, andthe crystal surface might deteriorate during that time. Therefore, theGaN layer 11 is grown beforehand at the same temperature as the quantumwell layer, and the In_(x)Ga_(1-x)N layer 12 is grown immediately afterthat, thereby, a MQW layer with higher quality can be obtained. Althoughthe In_(x)Ga_(1-x)N layer 12 is grown at 730° C. in the embodiment, itmay be grown at a temperature between 550° C. and 900° C. If thetemperature belows 550° C., the decomposition efficiency of NH₃ becomesextremely low, and droplets are easily formed. If the temperatureexceeds 900° C., the crystal growth of InGaN is difficult, due tothermal decomposition. The crystal growth temperature for obtaining anIn composition ratio necessary for obtaining blue or green lightemission is preferably 700° C. or higher, but is lower than 850° C. Thewidth of the In_(x)Ga_(1-x)N layer 12 is preferably in the range of 1 nmto 10 nm, and more preferably in the range of 1.5 nm to 5 nm.

Next, as shown in step S2 of FIG. 3, the supply of the raw material gasof TMIn is stopped, and a GaN layer 13 of 3 nm in film thickness isgrown at the susceptor temperature of 730° C. The GaN layer 13 may notbe grown. Where the GaN layer 13 is grown before a temperature rise,however, decomposition of the In_(x)Ga_(1-x)N layer 12 can be restrainedat the time of the later described temperature rise, and a MQW layerwith higher quality is obtained.

As shown in S3 of FIG. 3, the susceptor temperature is then raised to850° C. Although the crystal growth of the GaN layer 13 is suspended atthe time of the temperature rise in the embodiment, the temperature maybe raised while the GaN layer 13 is being grown.

As shown in step S4 of FIG. 3, a GaN layer 14 of 3 nm in film thicknessis then grown, while 250 sccm of H₂ gas as well as the carrier gas of N₂and the raw material gases of TMGa and NH₃ are being supplied to thereacting furnace. At this point, the supply of the N₂ gas is reduced bythe amount of the H₂ gas added, so that the total gas supply becomes39000 sccm. In this example, the supply of the N₂ gas is 20150 sccm(=20400−250). The flow rate of the H₂ gas is preferably between 0.01%and 50% of the total flow rate. If the flow rate of the H₂ gas is lowerthan 0.01%, the effect to remove clusters by virtue of the addition ofH₂ gas becomes smaller. If the flow rate of the H₂ gas is higher than50%, the In_(x)Ga_(1-x)N layer 12 is decomposed, and pits become larger.The grown film thickness of the GaN layer 14 with H₂ gas added varieswith the amount of H₂ gas added, and may have any value as long as itcontributes to removal of clusters and does not enlarge the pits.However, the film thickness of the GaN layer 14 is preferably 5 nm orsmaller. If the film thickness of the GaN layer 14 is larger than 5 nm,the thickness of the barrier layer becomes too large, and putsrestrictions on the design of the MQW structure. It is considered thatthe film thickness of the GaN layer 14 should be substantiallyequivalent to the height of clusters, but the present invention is notlimited to that.

After that, as shown in step S5 of FIG. 3, while the susceptortemperature is maintained at 850° C., the supply of the H₂ gas isstopped, and the carrier gas is exclusively the N₂ gas. A GaN layer 15of 9.5 nm in film thickness is then grown. Although the supply of the H₂gas is stopped when the GaN layer 15 is grown, a H₂ gas having a totalflow rate of less than 0.01% may be supplied. Also, the GaN layer 15 hasany film thickness as long as it is thick enough to fill the pits andcontributes to flattening. However, the film thickness of the GaN layer15 should preferably be in the range of 3 nm to 20 nm. A great filmthickness can effectively fill the pits and contribute to flattening thesurface. However, a great film thickness puts restrictions on the designof the well layer. Although the growth temperature of the GaN layer 15to be a barrier layer is 850° C. in the embodiment, it should only behigher than the growth temperature of the quantum well layer, and may bein the range of 800° C. to 1200° C., more preferably 850° C. to 950° C.In the range of the temperature, if the growth temperature of the GaNlayer 15 is higher than the growth temperature of the well layer by 50°C. or more, high crystal quality is easily obtained. If the growthtemperature is lower than 800° C., pits become larger during the growth,and it is difficult to obtain a flat film. If the growth temperature ishigher than 950° C., it takes time for the temperature to rise, and thecrystal quality of the In_(x)Ga_(1-x)N layer 12 becomes poorer due todecomposition or the like. If the crystal growth temperature is higherthan 1200° C., it exceeds the decomposition temperature of the GaN layer15, and is not suitable as a crystal growth temperature. The barrierlayer should be a nitride semiconductor having higher bandgap energythan the well layer, and may be doped with impurities. If the barrierlayer is formed with GaN, the crystal quality becomes high, and acrystal growth can be easily achieved.

As shown in step S6 of FIG. 3, the susceptor temperature is againlowered to 730° C. At this point, the supply of the raw material gas ofTMGa is stopped in the embodiment. After that, the raw material gas ofTMGa is supplied to the reacting furnace, and the above described GaNlayer 11 is grown (step S7 of FIG. 3). The procedures of steps S1through S6 are repeated. The procedures for growing the GaN layer 11through the GaN layer 15 are repeated four times, to form the MQW layer6.

Although GaN layers are used as the barrier layers 14 and 15 in theembodiment, In_(y)Ga_(1-y)N (0≦y<1, y<x) layers or In_(z)Ga_(1-z)N(0≦z<1, z<x) layers each having a lower In composition ratio than theIn_(x)Ga_(1-x)N (0<x≦1) layer 12 may be used to achieve the same effectsas those of the embodiment. Here, y and z are preferably 0.1 or smaller,to avoid relaxation of strain.

Next, Comparative Examples 1 through 4 of the embodiment is described.Each Comparative Example involves the same procedures as those of theembodiment, except for the formation of the MQW layer 6. Therefore, onlythe formation of the MQW layer will be described in the following.

Comparative Example 1

The crystal growth method of Comparative Example 1 is the same as thecrystal growth method of the embodiment, except that the procedures forgrowing the GaN layer 11, the GaN layer 13, and the GaN layer 14 shownin FIG. 2 are skipped in the crystal growth of the MQW layer, and afterthe In_(x)Ga_(1-x)N (0<x≦1) layer 12 to be the quantum well layer isgrown, the GaN layer 15 is grown at the temperature (=730° C.) at whichthe In_(x)Ga_(1-x)N (0<x≦1) layer 12 is crystal-grown, without a supplyof a H₂ gas. With the procedures for stacking the In_(x)Ga_(1-x)N(0<x≦1) layer 12 and the GaN layer 15 being one cycle, four cycles ofthose stacking procedures are repeated, to form the MQW layer.

Comparative Example 2

The crystal growth method of Comparative Example 2 is the same as thecrystal growth method of the embodiment, except that the procedures forgrowing the GaN layer 11 and the GaN layer 14 shown in FIG. 2 areskipped in the crystal growth of the MQW layer, and the GaN layer 13,the In_(x)Ga_(1-x)N (0<x≦1) layer 12 to be the quantum well layer, andthe GaN layer 15 are sequentially grown through the same procedures asthose of the embodiment. With the procedures for stacking the GaN layer13, the In_(x)Ga_(1-x)N (0<x≦1) layer 12, and the GaN layer 15 being onecycle, four cycles of the stacking procedures are repeated, to form theMQW layer.

Comparative Example 3

The crystal growth method of Comparative Example 3 is the same as thecrystal growth method of the embodiment, except that, with the stackingprocedures for growing the GaN layers 14 and 15 shown in FIG. 2 at agrowth temperature of 730° C., which is the same as the growthtemperature of the well layer, instead of 850° C. by further adding H₂gas being one cycle, four cycles of the stacking procedures are repeatedto form the MQW layer in the crystal growth of the MQW layer.

Comparative Example 4

The crystal growth method of Comparative Example 4 is the same as thecrystal growth method of the embodiment, except that, with the stackingprocedures for growing the GaN layer 15 shown in FIG. 2 at 850° C. whileadding H₂ gas being one cycle, four cycles of the stacking proceduresare repeated to form the MQW layer in the crystal growth of the MQWlayer. The sequences of the materials, the carrier gas, and thetemperatures in one cycle of the crystal growth of the MQW layer 6 inComparative Example 4 are therefore shown as steps S11 through S16 ofFIG. 4.

After the MQW layers were grown by the crystal grown methods of theembodiment, Comparative Example 1, Comparative Example 2, ComparativeExample 3, and Comparative Example 4, the samples were taken out of thereacting furnace, and the surfaces of the MQW layers were observed withan atomic force microscope (AFM). FIGS. 5 through 9 show the photographsof the observation results. As can be seen from the observation results,the RMS (Root Mean Square) value of the surface height indicatingflatness was 0.90 nm in the embodiment, 1.22 nm in Comparative Example1, 1.09 nm in Comparative Example 2, 1.99 nm in Comparative Example 3,and 1.02 nm in Comparative Example 4. As can be seen from theobservation results, the embodiment achieved the flattest surface. Thetemperature of the barrier layer was made higher in Comparative Example2 than Comparative Example 1. As can be seen from the results, the RMSvalue became smaller because of the increase in the temperature of thebarrier layer, and the surface was flattened. In Comparative Example 3,H₂ gas was added during the growth of the barrier layer, as opposed toComparative Example 1. However, the concave portions in the surface seenin Comparative Example 1 were even larger in Comparative Example 3, andthe surface flatness became poorer due to the addition of H₂ gas, thoughclusters were removed. In Comparative Example 4, the temperature wasmade higher during the growth of the barrier layer, and H₂ gas wasadded. When the surface was observed, the surface was flattened by thetemperature increase, and clusters were removed by the addition of H₂gas. The surface observed in Comparative Example 4 was flatter than thatin Comparative Example 1. However, since the addition of H₂ gas iscontinued through the growth of the GaN barrier layer in ComparativeExample 4, deep pits were observed with a transmission electronmicroscope. If a LED is manufactured with such a structure, a currentleaks through the deep pits, and hinders light emission from the quantumwell layer.

In the embodiment, on the other hand, after clusters are removed byadding H₂ gas, a GaN barrier layer is grown at a high temperaturewithout the addition of H₂ gas, to form a flat surface. In this manner,the two effects are appropriately achieved at the same time.

This combination of advantageous effects has not been suggested inconventional arts. Compared with Comparative Examples, since GaN barrierlayers are grown at a higher temperature, surface migration of thematerials at the time of crystal growth is accelerated. Accordingly,pits are effectively filled. Also, H₂ gas has the effect to performetching on InGaN, and it is considered that clusters with high Incomposition ratios are selectively etched by virtue of H₂ gas. Withthose effects being taken into consideration, clusters are removed byadding H₂ gas after the growth of an InGaN layer as the quantum welllayer. After that, a GaN layer is grown at a high temperature withoutthe addition of H₂ gas. In this manner, the surface can be flattened.

The samples of the embodiment and Comparative Examples 1 through 4 werereturned into the reacting furnace, and GaN layers were grown at 1030°C. for ten minutes. The conditions that GaN layers are grown at 1030° C.for ten minutes match for forming LED structures the temperature andtime conditions for growing a p-type layer on a MQW layer. Therefore,pseudo LED structures were formed. Those samples were then subjected tophotoluminescence (PL) measurement. The results are shown in FIG. 10. Ascan be seen from FIG. 10, the emission intensity from the sample of theembodiment is the highest.

As described above, according to the embodiment, clusters can be removedby adding H₂ gas, and a GaN layer is then formed without the addition ofH₂ gas. In this manner, flattening can be performed through a relativelyhigh-temperature growth. Accordingly, generation of clusters can berestrained, and enlargement of pits can be prevented. As a result, ahigh-quality nitride semiconductor can be obtained. Also, a crystalgrowth method for nitride semiconductors that achieves high-efficiencylight emission with a high luminance can be achieved, and such a methodhas great industrial applicability. The crystal growth method of theembodiment can also be applied to the crystal growth of a periodicstructure such as the crystal growth of a superlattice layer or amultilayer reflector structure (a distributed bragg reflector (DBR))that is designed to control strain and reduce threading dislocationdensity. This embodiment may be applied not only to LEDs, but also tolaser diodes (LDs) and light receiving devices.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel methods and systems describedherein may be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the methods andsystems described herein may be made without departing from the spiritof the inventions. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fall within thescope and spirit of the inventions.

1. (canceled)
 2. A crystal growth method for nitride semiconductors,comprising: growing a first semiconductor layer containingIn_(x)Ga_(1-x)N (0<x≦1) on a substrate, with the use of a first carriergas formed with an inert gas; growing a second semiconductor layercontaining In_(y)Ga_(1-y)N (0≦y<1, y<x) on the first semiconductorlayer, with the use of a second carrier gas containing the inert gas andH₂ gas, an amount of the H₂ gas being smaller than an amount of theinert gas; and growing a third semiconductor layer containingIn_(z)Ga_(1-z)N (0≦z<1, z<x) on the second semiconductor layer, with theuse of a third carrier gas containing the inert gas and H₂ gas, anamount of the H₂ gas in the third carrier gas being a smaller than theamount of H₂ gas in the second carrier gas.
 3. The method according toclaim 2, wherein the third semiconductor layer has substantially thesame composition as the second semiconductor layer.
 4. The methodaccording to claim 2, wherein procedures for growing the firstsemiconductor layer, the second semiconductor layer, and the thirdsemiconductor layer are set as one cycle, and a plurality of cycles eachbeing the one cycle are repeated.
 5. The method according to claim 2,further comprising growing a fourth semiconductor layer containingIn_(u)Ga_(1-u)N (0≦u<1, u<x) with the use of the first carrier gasbefore the first semiconductor layer is grown.
 6. The method accordingto claim 2, further comprising growing a fifth semiconductor layercontaining In_(v)Ga_(1-v)N (0≦v<1, v<x) with the use of the firstcarrier gas after the first semiconductor layer is grown but before thesecond semiconductor layer is grown.
 7. The method according to claim 2,wherein the second and third semiconductor layers are GaN layers.
 8. Themethod according to claim 2, wherein the first semiconductor layer isgrown at a first growth temperature of 700° C. or higher but not higherthan 850° C., the second semiconductor layer is grown at a second growthtemperature higher than the first growth temperature, and the thirdsemiconductor layer is grown at a third growth temperature higher thanthe first growth temperature but not higher than the second growthtemperature.
 9. The method according to claim 8, wherein the thirdgrowth temperature is higher than the first growth temperature by 50° C.or more.
 10. The method according to claim 8, wherein the third growthtemperature is 800° C. or higher but not higher than the 950° C.